This invention relates to a double sided printed circuit board and to a method for making double sided printed circuit boards.
Printed circuit boards are commonplace in modern society. Resistors, capacitors, semiconductors and integrated circuits are joined together by conductive paths on an insulating surface. The board provides both the support for the circuit elements and the means for conductance between them. Where space is limited or the circuitry complex, both sides of a board might be used, or multiple boards might be joined together.
Manufacture of a printed circuit board generally begins with a substratum of insulating material such as bakelite, epoxy resin, glass fiber reinforced resins, or polyester. Using substractive technology well known in the prior art, a lamina of copper or other conductor is adhered to the substratum. Then, using a printing or photographic process, the desired circuit pattern is outlined on the lamina. Finally, unwanted material is removed by etching or routing, leaving the circuit in place.
The prior art also provides other methods for deposing a circuit on a printed circuit board, including electroplating and electroless or wet chemical plating, involving the use of catalysts. For large boards, such as those used in particle accelerator calorimeters, these chemical methods are less desirable because they require submersion, photographic exposure, or offset printing of the full face of the board, and therefore large equipment investments. For large boards, mechanical methods are preferred, such as using a router guided by a computer.
The choice of methods has not been so clear for the next step in the manufacture of very large printed circuit boards, namely provision for conductance between double sides. Typically conductance between the sides of a printed circuit board is provided by means of a metal lined through-hole or by a component such as a resistor or capacitor mounted within a metal lined through-hole. A metallic land surrounds the lined hole, providing a connection between the conductive path or component mounting pin and the metal lining.
Several methods for the metallization of the sides of the through-hole and the creation of the land are well known in the prior art. An eyelet or grommet can be inserted, and soldered in place. Or, the hole can be lined as part of the electroplating process; the inside of the hole is made electrically conductive with graphite or a thin metallic film before the board is immersed in the electroplating bath. Dry metallization techniques such as vacuum metallization have also been developed, including sputtering, ion plating and electron beam vacuum deposition. Examples of the prior art include disclosures contained in U.S. Pat. Nos. 4,610,756, 4,360,570, and 4,351,697.
One problem associated with these methods for metallizing a hole is that adhesion of the solder or metallic liner to the laminae is often less than adequate. Also, holes must be drilled precisely, free of burrs and errant fibers. Pre-treatment of the surface with suitable chemicals is often required to insure that liner coverage is adequate and uniform to provide a good electrically conductive surface. Clean-up is frequently necessary to remove superfluous material.
The above problems are accentuated when the printed circuit board is extraordinarily large or highly miniaturized, or when it is to be used in restrictive environments such as during extreme temperature changes or while carrying large currents. The costs of overcoming these problems can be high, including specialized handling and cleaning equipment, expensive materials and chemicals, and labor intensive pre-treatment and clean-up operations.
It is an object of this invention to provide a double-sided circuit board which maintains superior conductance between opposite sides of the board.
It is another object of this invention to provide a double-sided circuit board which substantially reduces or eliminates problems of adhesion within through-holes, and resultant costs for pre-treatment and clean-up.
It is another object of this invention to provide a double-sided circuit board which is resilient in extreme conditions, such as during extreme temperature changes and while carrying large currents.
It is a further object of this invention to provide a method for economically manufacturing a double-sided printed circuit board, using equipment which is readily available at low cost.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.